Our invention relates to the conversion of electric signals from digital to analog form. More specifically, our invention concerns a method of, and means for, translating a digitized audio signal or like digital data signal into an equivalent analog signal with the addition of dither (an artificially created white noise signal) to the digital data signal and subsequent removal of the dither from the analog data signal for the reduction of noise and distortion.
The pulse code modulation (PCM) or digital processing of audio signals has become, or is becoming, the mainstream of high fidelity sound recording and reproduction with the advent and ever increasing commercial acceptance of compact discs (CDs). There are, however, some problems left unsolved in the area of digital sound processing. One of these is the "quantization noise", that is, the differences between the samples of the music wave and the quantized values of the samples. The quantization noise becomes particularly pronounced, distorting the reproduced sound as higher harmonics, when the input signal level is low and there are only a small number of quantization steps. Even when the input signal level is high, the quantization noise will distort the signal if it changes slowly.
A conventional solution to this quantization noise problem has been the use of dither, intended to turn the quantization noise into white noise which hardly affects the appreciation of the reproduced sound. A digital dither signal is superposed on the audio signal prior to its digital to analog conversion and is removed from the analog audio signal following the conversion. This technique is disclosed for example in the article entitled "The Application of Large Amplitude Dither to the Quantization of Wide Range Audio Signals" by Yoshio Yamasaki in The Journal of the Acoustical Society of Japan, Vol 39, No. 7, published 1983.
We will now discuss two familiar examples of digital to analog conversion systems relying on dither for the suppression of quantization noise. One such known system adds a dither signal, digitized by an analog to digital (A/D) converter on being put out by a dither generator, to a digital audio or data signal. The data and dither are then converted by a digital to analog (D/A) converter into analog form. Then the analog data and dither signal is fed through a low pass filter into a subtracter circuit, to which is also supplied the analog dither signal directly from the dither generator for subtracting the analog dither from the analog data and dither signal. We object to this known system because of the time difference between the dither fed directly from the dither generator to the substracter circuit and the dither delivered thereto with the data signal via the A/D converter, adder, D/A converter, low pass filter, and substracter. The time difference makes impossible the complete removal of the dither from the data signal.
Another known system employs an additional D/A converter which has its input connected to the dither generator via the A/D converter and its output to the subtracter, with a view to the elimination of the noted time difference. The low pass filter is connected to the output of the subtracter, instead of to its input as in the first recited prior art system. Thus, on being put out by the dither generator, the dither is fed to one input of the subtracter via the A/D converter, adder, and first D/A converter on one hand and, on the other hand, to the other input of the subtracter via the A/D converter and second D/A converter. Theoretically, the dither signals to both inputs of the subtracter are in phase, so that the dither is thoroughly removable from the analog data signal. In practice, however, it is nearly impossible to provide two D/A converters of exactly the same constructional details and the same performance characteristics. The different converters will give rise to different conversion errors, with the result that some dither component remain unremoved from the analog data signal to manifest itself as noise or distortion upon sound reproduction.